An integrated circuit is generally operable over a range of an input bias voltage, such as over a range of a maximum rated and a minimum rated input bias voltage. Circuitry on the integrated circuit such as a control chip for a power converter will not operate reliably at an input bias voltage lower than the minimum rated voltage, and the circuitry can be irreversibly damaged at an input bias voltage higher than the maximum rated voltage. As a protective feature, an undervoltage detector is usually included in the design of the integrated circuit to generate a start current for the chip and to set a lowest working input bias voltage threshold for the chip. If the input bias voltage is less than the threshold, operation of the chip is disabled.
The input bias voltage supplied to an integrated circuit is generally sensed by an undervoltage detector employing a resistor-divider network coupled in series with a number of transistors, each transistor connected in a diode configuration by coupling its base to its collector or its gate to its drain. Temperature- and process-dependent voltage drops across diodes generally exhibit a different variation from corresponding voltage drops across resistors, particularly resistors formed on the integrated circuit as semiconductor elements. The different behaviors of these voltage drops produce a variable temperature- and process-dependent threshold voltage in a comparator employed to detect a lower limit for the input bias voltage. The variability in the threshold voltage generally requires accommodation in the design of the undervoltage detector that limits a lower operational voltage range for the integrated circuit.
Turning now to FIG. 1, illustrated is a schematic drawing of a conventional undervoltage detector for an integrated circuit that may be employed to set a lowest working input bias voltage threshold for the integrated circuit. If an input bias voltage is less than the threshold, operation of the integrated circuit is disabled, and the integrated circuit is maintained in a standby state. If the input bias voltage is greater than the threshold, normal operation of the integrated circuit is enabled.
The input bias voltage VBAT is sensed by an undervoltage detector employing a resistor-divider network. A scaled value V1 of the input bias voltage VBAT produced at the junction of resistors R1 and R2 is sensed by a comparator formed with transistors M1 and M2 coupled to a current mirror formed by transistors M3 and M4. A reference voltage VBG is supplied to the comparator by a bandgap reference, such as a 1.25 V bandgap reference. The design of current mirrors is well known in the art, and will not be further described in the interest of brevity. The output signal 103 of the comparator is inverted by transistor M7, which is then sensed by inverter INV1 and inverted again by inverter INV2 to produce an undervoltage detector output signal UPM1 dependent on a chip turn-on threshold voltage.
Providing a wide operational voltage range for an integrated circuit is often a key success factor for such devices in the marketplace. The design of an improved arrangement for an integrated circuit to detect an input bias voltage below an undervoltage limit would address an unresolved application need.